1. Field of the Invention
The present invention relates to improvements in and relating to protective devices for photographic masks or reticles used in the fabrication of integrated circuits.
2. Description of the Prior Art
In the past, in order to protect the surface of photographic masks or reticles from dust, devices have been known in which, for example, a transparent thin film referred to as a pellicle is attached over one open surface of a frame of a size sufficient to enclose the pattern area on a substrate of a mask and the other open surface of the frame is made fast with an adhesive or adhesive tape to the substrate in such a manner that the pattern area is included within the frame. In this device, the pellicle is supported by the frame at a predetermined distance from the pattern on the surface of the substrate.
Such protective pellicle has the function of preventing the deposition of dust on the substrate of a mask and protecting the mask and it contributes toward improving the productivity of integrated circuits, etc.
For instance, if dust deposits on a photographic mask for semiconductor fabrication purposes, there is the danger of the pattern of the dust being projected onto a semiconductor wafer during the photolithographic operation and thereby deteriorating the reliability of the resulting semi-conductor devices due to operational malfunctions, etc. Then, by using the previously mentioned protective device, it is possible to shut off the atmosphere on the mask from the outside air and thereby alleviate the deficiencies.
On the other hand, if any foreign particles such as dust deposit on the surface of a mask substrate having a pellicle already mounted thereon or to the inner surface of the pellicle, the pellicle must be removed from the substrate to clean the substrate or the pellicle again. However, the pellicle mounted on the mask substrate by the frame cannot be easily removed from the mask substrate, with the resulting inconveniences that the surface of the substrate is damaged during the removing, that the removed pellicle becomes unsuitable for reuse and so on.
Also, while the frame of the pellicle must be aligned with the mask substrate so as to enclose the pattern area, if the position of the frame is beyond a predetermined error range, it is also necessary to remove the frame and to again adhere the frame in the proper position. In such a case, the fixedly mounted pellicle frame is also inconvenient.
Further, the space enclosed by the mask substrate, the frame and the pellicle is in very excellently sealed condition hermetically. As a result, when the mask is transported by aircraft or transferred to a high altitude, the pressure difference between the internal pressure and the external pressure causes the pellicle attached to the frame to bulge or become depressed. Such deformation of the pellicle is disadvantageous in that not only the optical characteristics of the pellicle itself are changed, but also, depending on the degree of the deformation, the pellicle is caused to contact with the surface of the mask substrate or the pellicle is damaged by contacting with any metal part in the surroundings during the transport of the mask within a projection apparatus and so on.